Telephone System, Server, and Terminal Device

1. A telephone system which comprises a plurality of terminal devices configured to process a signaling protocol, and a server which transmits and receives signals using a first transfer protocol or a second transfer protocol as a lower layer protocol to form a session of the signaling protocol, wherein each of the plurality of terminal devices comprises: a request processor configured to transmit a request signal to form the session using the first transfer protocol, and wherein the server comprises: a determination module configured to determine whether or not the first transfer protocol is available for transmitting signals; a session forming module configured to form a session with a request source by setting a transfer protocol to either (i) the first transfer protocol when the determination module determines that use of the first transfer protocol is available or (ii) the second transfer protocol; a switching module configured to switch, when a communication using the first transfer protocol becomes available turns, the lower layer protocol of the communication with the terminal device using the second transfer protocol from the second transfer protocol to the first transfer protocol while maintaining the session already formed in the signaling protocol; and a storage device comprises a management table which assigns a session formation priority using the first transfer protocol to each terminal device; wherein the determination module (i) confirms, by referring to the management table when the request signal from the terminal device arrives, the presence or absence of a terminal device having a lower session formation priority than the terminal device as the request source and using the first transfer protocol, (ii) instructs the switching module to change the lower layer protocol of a session already formed with the terminal device having the lower session formation priority than the terminal device as the request source from the first transfer protocol to the second transfer protocol, and (iii) grants permission to the request signal from the terminal device as the request source when a resource is assured after the lower layer protocol is changed.

2. The system of claim 1 , wherein the request module continuously requests when the session is formed by the communication using the second transfer protocol, to change the transfer protocol to be used to the first transfer protocol while maintaining the session.

3. The system of claim 1 , wherein the terminal device further comprises a secure communication module configured to conduct secure communication via a session formed using the first transfer protocol.

4. The system of claim 3 , wherein the terminal device further comprises an informing module configured to inform a user that secure communication is allowed to be conducted.

5. The system of claim 1 , wherein the first transfer protocol is Transmission Control Protocol (TCP), and the second transfer protocol is User Datagram Protocol (UDP).

6. A server which transmits and receives signals using a first transfer protocol or a second transfer protocol as a lower layer protocol to form sessions of a signaling protocol with one or more terminal devices configured to process the signaling protocol, comprising: a determination module configured to determine, based on a request from a request source being a terminal device, whether or not the first transfer protocol is available for transmitting signals; a session forming module configured to form the session with the request source by setting a transfer protocol to either (i) the first transfer protocol when the determination module determines that use of the first transfer protocol is available or (ii) the second transfer protocol; a switching module configured to switch, when a communication using the first transfer protocol becomes available, the lower layer protocol of the communication with the request source using the second transfer protocol from the second transfer protocol to the first transfer protocol while maintaining the session already formed in the signaling protocol; and a storage device comprises a management table which assigns a session formation priority using the first transfer protocol to each terminal device; wherein the determination module (i) confirming, by referring to the management table when the request from the resource source arrives, the presence or absence of a terminal device having the lower session formation priority than the request source and using the first transfer protocol, (ii) instructing the switching module to change the lower layer protocol of a session already formed with the terminal device having the lower session formation priority than the request source from the first transfer protocol to the second transfer protocol, and (iii) granting permission to the request from the request source when a resource is assured after the lower layer protocol is changed.

7. A terminal device which is configured to process a signaling protocol with a server which transmits and receives signals using a first transfer protocol or a second transfer protocol as a lower layer protocol to form a session of the signaling protocol, comprising: a request processor configured to transmit a request signal to form the session using the first transfer protocol; and a module configured to continuously request to change the lower layer protocol to the first transfer protocol while the session with the server is formed using the second transfer protocol and the session is continued until the server confirms, after receiving the request signal from the terminal device and referring to a management table, which assigns a session formation priority using the first transfer protocol to one or more terminal devices, the presence or absence of a second terminal device having the lower session formation priority than the terminal device and using the first transfer protocol, changes the lower layer protocol of a session already formed with the second terminal device having the lower session formation priority than the terminal device from the first transfer protocol to the second transfer protocol, and grants permission to the request signal from the terminal device when a resource is assured after the lower layer protocol is changed.

8. The terminal device of claim 7 , further comprising: a secure communication module configured to conduct secure communication via a session formed using the first transfer protocol.

9. The terminal device of claim 8 , further comprising: an informing module configured to inform a user that secure communication is allowed to be conducted.